Device and method for pre-treating a signal to be transmitted using a non-linear amplifier with an upstream band-pass filter

ABSTRACT

A device for pretreating a signal (S 1 ) to be transmitted over a transmission path, wherein the transmission path comprises a band-pass filter ( 14 ) whose pass-band width is smaller than a predetermined bandwidth and a non-linear amplifier ( 16 ), includes a linearization means ( 20 ) for performing a linearization of the signal (S 1 ) to be transmitted before the transmission of the same, such that a non-linearity caused by the non-linear amplifier ( 16 ) is counteracted. Further, the device includes a signal-shaping means ( 22 ) downstream to the linearization means ( 20 ) comprising a frequency response such that the overall frequency response of the signal-shaping means ( 22 ) and the band-pass filter ( 14 ) corresponds to the frequency response of a filter whose pass-band width is at least equal to the predetermined bandwidth.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a device and a method forpretreating a signal to be transferred using a non-linear amplifierwhich is subjected to a prelinearization and a predistortion,respectively, in order to counteract non-linearities introduced by thenon-linear amplifier. In particular, the present invention relates to adevice and a method for pretreating a signal which is to be transferredover a non-linear amplifier which has a preceding narrow-bandedband-pass filter whose pass-band width is only slightly larger than thesignal bandwidth to be processed.

[0003] 2. Description of Prior Art

[0004] It is known to subject signals to a predistortion andprelinearization, respectively, which are to be transferred using anon-linear amplifier, for example using a non-linear amplifier which isarranged in a communication satellite, in order to counteract thenon-linearities generated by the amplifier and completely compensatethem in the optimum case, respectively. In the case of a satellitetransmission system it is attempted to compensate distortions this waywhich occur in the output amplifier of the satellite, by a correspondingpredistortion of the transmission signal in the ground station.

[0005] An exemplary method for generating a linearized amplifier outputsignal in which distortions generated by a non-linear amplifier arereduced and compensated, respectively, responding to a predistortedinput signal is described in U.S. Pat. No. 5,049,832. In this method theamount square of the input signal is detected, wherein a table isaccessed depending on the amount square in order to multiply the inputsignal with a predetermined complex coefficient in order to cause apredistortion of the input signal, such that the output signal amplifiedby the non-linear amplifier is linearized according to amount and phase.A means for comparing input signal and output signal is provided inorder to perform an adjustment of the coefficients stored in the table.

[0006] An alternative method for performing a predistortion for anon-linear transmission link in the high-frequency range is known fromEP 0885482 B1, wherein an envelope curve of an input signal is detectedand quantized in the method described there, in order to generatequantized envelope curve values. On the basis of detected, quantizedenvelope curve values a table means is accessed in which complexpredistortion coefficients are stored. The input signal is predistortedon the basis of the predistortion coefficients, such that the distortionintroduced by the non-linear transmission link is basically compensatedaccording to amount and phase.

[0007] A still further method for performing a predistortion andprelinearization, respectively, of a transmission signal to betransferred over a non-linear transmission link is disclosed in the notpre-published German Patent Application 19927952.7. In the methodillustrated there an estimation signal is generated using a table inwhich complex coefficients are stored, at first depending on an inputsignal, wherein the estimation signal illustrates an estimation of anoutput signal amplified by a non-linear amplifier. A difference isformed from the estimation signal and the input signal illustrating anerror signal illustrating the estimation of the error introduced by thenon-linear amplifier. This error signal is time-expanded by atime-dispersive element and then subtracted from the input signal inorder to perform a pre-linearization of the input signal, such that anerror signal part in the frequency spectrum of the output signal isshifted away from the useable frequency range of the signal.

[0008] The above-mentioned predistortion and prelinearization methods,respectively, and all further predistortion methods of a plurality knownin the art, wherein a predistortion and prelinearization, respectively,of a signal is executed before the transmission of the same by anon-linear amplifier, will fail when the bandwidth of a precedingchannel filter to the non-linear amplifier lies essentially in the rangeof the signal bandwidth to be processed or amounts to less than three tofive times the bandwidth of the undistorted signal, respectively.

SUMMARY OF THE INVENTION

[0009] It is the object of the present invention to provide a device anda method for pretreating a signal to be transmitted using a non-linearamplifier which also facilitate a sufficient prelinearizing andpredistortion, respectively, of the signal to be transmitted before itstransmission when a narrow-band band-pass filter is preceding thenon-linearized amplifier.

[0010] According to a first aspect of the invention this object isachieved by a device for pretreating a signal to be transmitted over atransmission path, wherein the transmission path is a band-pass filterwhose pass-band width is less than a predetermined bandwidth andcomprises a non-linear amplifier, comprising:

[0011] a linearization means for performing such a linearization of thesignal to be transmitted before the transmission which counteracts anon-linearity caused by the non-linear amplifier; and

[0012] a signal-shaping means downstream to the linearization meanscomprising such a frequency response that the overall frequency responseof the signal-shaping means and the band-pass filter corresponds to thefrequency response of a filter whose pass-band width is at least equalto the predetermined bandwidth.

[0013] According to a second aspect of the invention, this object isachieved by a method for transmitting a first signal over a transmissionpath comprising a band-pass filter whose pass-band width is smaller thana predetermined bandwidth and a non-linear amplifier, comprising thefollowing steps:

[0014] performing such a linearization of a first signal that anon-linearity caused by the non-linear amplifier is compensated in orderto generate a signal;

[0015] directing the second signal through a signal-shaping meanscomprising such a frequency response that the overall frequency responseof the signal-shaping means and the band-pass filter corresponds to thefrequency response of a filter whose pass-band width is at least equalto a predetermined bandwidth for generating a third signal; and

[0016] transmitting the third signal over the transmission path.

[0017] The present invention is primarily based on the findings thatwith all known methods for pre-distorting and prelinearizing,respectively, of signals to be transmitted over non-linear transmissionlinks energy outside the useable band is added to the signals to betransmitted over the predistortion and prelinearization, respectively.This energy, however, is filtered and strongly reduced, respectively, bynarrow-banded band-pass filters preceding the non-linear amplifier, sothat the predistortion and prelinearization, respectively, can not causethe desired effect to reduce the non-linearity introduced by thenon-linear amplifier and compensate it in the optimum case,respectively. In order to facilitate that this energy reaches thenon-linear amplifier basically completely outside the useable band inorder to cause a linearization of the output signal of the same, asdesired, a signal-shaping means is provided according to the invention.The signal-shaping means comprises such a frequency response that theoverall frequency response of a serial interconnection of thesignal-shaping means and the band-pass filter downstream to thenon-linear amplifier comprises such a pass-band width that apart fromthe signal parts in the useable band also the energy parts necessary fora linearization added by the predistortion and prelinearization,respectively, may reach the non-linear amplifier outside the useableband without an excessive attenuation.

[0018] If it is assumed that energy parts are introduced within afrequency band by the predistortion and prelinearization, respectively,which is approximately three times as wide as the useable bandwidth ofthe signal to be transferred, then the frequency response of thesignal-shaping means is preferably selected such that the overallfrequency response of the signal-shaping means and the band-pass filtercorresponds to the frequency response of a filter whose pass-band widthis at least three times as large as the signal bandwidth. Preferably,the frequency response of the signal-shaping means is set such that theoverall frequency response of an interconnection of the signal-shapingmeans and the band-pass filter corresponds to the frequency response ofa band-pass filter whose pass-band width corresponds to three to fivetimes the signal bandwidth.

[0019] The invention may preferably be used in the area of satellitecommunication if conventional “transparent” transponders are used on thesatellite whose output amplifiers cause non-linear distortions and whichare equipped with a narrow-band band-pass filter preceding the outputamplifier and whose bandwidth is only slightly larger than the signalbandwidth to be processed. The amplifiers in such communicationtransponders are usually travelling-wave tubes causing significantdistortions.

[0020] By using conventional linearization methods the inventionfacilitates the linearization of the output amplifier of a satelliteeven if the channel filter preceding the same would not allow alinearization according to conventional methods. This is achieved by theinventive signal-shaping means being downstream to a conventionallinearization means in the ground station. Thereby a significantreduction of spurious emission is reached by the output amplifier of thesatellite which again allows a higher modulation and thereby a highertransmission power of the satellite.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In the following, preferred embodiments of the present inventionare described in more detail referring to the accompanying drawings, inwhich:

[0022]FIG. 1 shows a rough schematical illustration of a satellitetransmission system wherein one embodiment of the present invention isimplemented;

[0023]FIG. 2 shows a schematical diagram which shows the frequencyresponses for explaining the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0024] Referring to FIG. 1 an embodiment of the present invention isdescribed in the following wherein it is used for a satellitetransmission system.

[0025] In FIG. 1 a ground station 10 and a communication satellite 12 ofa satellite transmission system are illustrated schematically. Theinventive device for pretreating a signal to be transmitted over atransmission path is thereby implemented in the ground station 10 of thesatellite transmission system.

[0026] Existing communication satellites 12, so-called “transparent”transponders, usually comprise a channel band-pass filter 14 which ispreceding a non-linear amplifier 16. The bandwidth of the channelband-pass filter 14 is usually only slightly larger than the signalbandwidth of a signal to be transmitted before a predistortion and aprelinearization, respectively, of the same. The amplifier 16 of thetransponder and the communication satellites 12, respectively, isusually a travelling wave tube causing significant distortions.

[0027] The ground station 10 includes a signal source 18 generating asignal S1 to be transmitted. At this point it is to be noted that inexisting systems the pass-band width of the channel band-pass filter 14in the communication satellite 12 is adjusted to the signal bandwidth ofthe input signal S1 and is slightly larger than the same, respectively.

[0028] A linearization means 20 is connected to the output of the signalsource 18. The linearization means 20 may be a conventionallinearization and predistortion means, respectively, as was describedabove referring to the prior art. The linearization means 20 generates apredistorted input signal S2 from the input signal S1. According to theinvention, the output of the linearization means 20 is connected to asignal-shaping means which generates an overall frequency responsetogether with the channel band-pass filter 14 in the communicationsatellite 12 comprising a bandwidth which is three times as large as thesignal bandwidth of the input signal S1. The output signal S3 of thesignal-shaping means 22 is then transmitted to the communicationsatellite 12, filtered through the channel band-pass filter 14 anddirected to the output amplifier of the satellite 12 as the filteredsignal S4.

[0029] As the overall frequency response of the combination ofsignal-shaping means 22 and channel band-pass filter 14 comprises abandwidth which is at least three times as large as the signalbandwidth, energy which is added by the predistortion outside the usefulband is not attenuated stronger by this overall frequency response thansignal parts present in the useful band. Referring to the frequencyresponse of the signal-shaping means 22 and the channel band-pass filter14 as well as regarding the overall frequency response of aninterconnection of these elements reference is made to the followingdescription of FIG. 2.

[0030] An optional test receiver 24 which may be provided in the groundstation 10 or separate from the same receives the output signal S5 ofthe communication satellite 12 and thereby facilitates to set thecharacteristic of the linearization means 20 and the signal-shapingmeans 22 optimally over a comparison of the input signal S1 to theoutput signal S5, as is schematically illustrated by the arrows 26 inFIG. 1. In particular, for example predistortion coefficients of thelinearization means and coefficients of the signal-shaping means are forexample set and adapted by this, respectively, in order to generate thedesired frequency response.

[0031] In FIG. 2 a diagram of the frequency response 30 of the channelband-pass filter 14, the frequency response 32 of the signal-shapingmeans 22 and the overall frequency response 34 resulting from a seriesconnection of the signal-shaping means 22 and the channel band-passfilter 30 are shown.

[0032] On the abscissa of FIG. 2 the standardized frequency isillustrated starting from a center frequency 0 of the signal bandwidth.On the ordinate of FIG. 2 standardized attenuation and amplificationvalues are entered, respectively, wherein it is clear that therespective frequency, attenuation and amplification values actuallypresent are dependent on the used frequency band and the used members,respectively.

[0033] Referring to FIG. 2 it is assumed that the pass-band width of afrequency response 30 of the channel band-pass filter 14 corresponds tothe signal bandwidth of the input signal or is only slightly larger thanthe same, respectively. Due to such a low bandwidth of the channelband-pass filter 14 energy parts generated by the linearization means 20outside the useful band would be filtered out, so that the linearizationas desired may not be realized. Therefore, the signal-shaping means 22is provided according to the invention, comprising a frequency response32 which is generally “inverse” and “complementary”, respectively, tothe frequency response 30, such that a combination of the two frequencyresponses, i.e. an addition of the same, caused by a connection inseries of the elements 22 and 14, generates an overall frequencyresponse 34 with a pass-band width of at least three times the signalbandwidth. The signal-shaping means 22 may thereby also be referred toas an inverse band-pass filter.

[0034] In order to generate the frequency response 32 illustrated inFIG. 2 the signal-shaping means 22 may consist of a suitable amplifiercircuit having an amplification complementary to the attenuation in thefrequency ranges in which the channel band-pass filter 14 provides anattenuation. Thus, the overall frequency response 34 is generated, as itis shown in FIG. 2, comprising a pass-band width, wherein the energyoutside the useful band added by the predistortion remains basicallyunattenuated or is not attenuated stronger than the signal within theuseful band, respectively.

[0035] The present invention thus facilitates the use of knownlinearization methods also in the field of satellite communication usingexisting satellites, wherein a narrow-banded band-pass filter ispreceding the output amplifier.

[0036] Finally, it is noted that the pass-band width of the overallfrequency response generated by the signal-shaping means and the channelband-pass depends on the fact in which frequency ranges theprelinearization means generates energy parts which are necessary inorder to generate a sufficiently linearized signal at the output of theamplifier. In conventional predistortion methods it is usuallysufficient to set the pass-band width of the overall frequency band tothree to five times the signal bandwidth. Depending on the predistortionused also a less wide pass-band width of the overall frequency responsemay suffice, however, or a broader overall pass-band width may beneeded, respectively.

What is claimed is:
 1. A device for pretreating a signal to betransmitted over a transmission path, wherein the transmission pathcomprises a band-pass filter whose pass-band width is smaller than apredetermined bandwidth and a non-linear amplifier, comprising: alinearizer for performing such a linearization of the signal to betransmitted before the transmission of the same, that a non-linearitycaused by the non-linear amplifier is counteracted; and a signal shaperdownstream to the linearizer comprising such a frequency response thatthe overall frequency response of the signal shaper and the band-passfilter corresponds to the frequency response of a filter whose pass-bandwidth is at least equal to the predetermined bandwidth.
 2. The deviceaccording to claim 1, wherein the predetermined bandwidth corresponds tothree times the signal bandwidth of the signal to be transmitted.
 3. Thedevice according to claim 1, wherein the linearizer and the signalshaper are arranged in the ground station of a satellite transmissionsystem and wherein the band-pass filter and the non-linear amplifier arearranged in a satellite of the satellite transmission system.
 4. Thedevice according to claim 1, further comprising a receiver for receivinga signal to be transmitted over the transmission path and a means forsetting coefficients of the linearization means and/or thesignal-shaping means based on the received transmitted signal and thesignal to be transmitted.
 5. A method for transmitting a first signalover a transmission path comprising a band-pass filter whose pass-bandwidth is less than a predetermined bandwidth, and a non-linearamplifier, comprising the following steps: performing such alinearization of the first signal that a non-linearity caused by thenon-linear amplifier is compensated for, in order to generate a secondsignal; directing the second signal through a signal shaper comprisingsuch a frequency response that the overall frequency response of thesignal shaper and the band-pass filter corresponds to the frequencyresponse of a filter whose pass-band width is at least equal to thepredetermined bandwidth, in order to generate a third signal; andtransmitting the third signal over the transmission path.